1. Field of the Invention
The present invention pertains to a method of obtaining a rounded top corner on a trench formed in a semiconductor substrate.
2. Brief Description of the Background Art
Trenches formed in silicon using traditional etching methods typically have sharp, squared-off top corners. These sharp, squared-off corners lead to high field stress in film layers subsequently deposited thereon during further processing steps. The high field stress can potentially lead to the breakdown of the deposited film layers. The sharp, squared-off corners are a point of charge accumulation, which can cause the charge to pass through the insulating (dielectric) material, resulting in eventual device failure. As such, rounding of the top trench corner can be critical for device performance, particularly over the long term.
A prior art method for "rounding" the top corners in shallow silicon trench applications is shown in FIG. 1. In the method illustrated in FIG. 1, a film stack 2 comprising a patterned layer 8 of photoresist, a layer 6 of silicon nitride, and a layer 4 of silicon dioxide is deposited (from top to bottom) on a silicon substrate 10. As shown in FIG. 1A, the film stack 2 is first isotropically dry etched to form a trench 12 in the silicon substrate 10.
The film stack is then wet etched using a conventional wet etch solution (such as hydrogen fluoride, ammonium fluoride, or a buffered oxide etch solution) to undercut the silicon dioxide layer 4 and expose the sharp top corners 14 of the silicon trench 12, as shown in FIG. 1B. FIG. 1B-2 is an enlarged view of a top corner 14 of silicon trench 12.
A secondary oxide layer 16 is then formed by introducing oxygen into the processing chamber, where it reacts with exposed silicon substrate to form a layer of silicon dioxide lining the trench. As shown in FIG. 1C, the top trench corners 18 formed by growth of the secondary oxide layer 16 are rounded. However, although the silicon dioxide corner is rounded, there is still an underlying corner 20 on the silicon surface which can serve as a point of charge accumulation and, ultimately, device failure.
Therefore, as described above, this prior art method does not succeed in sufficiently rounding the top corners of the silicon trench. In addition, there are other disadvantages to this method. For example, during the formation of the secondary oxide layer 16, some of the silicon substrate material 10 is consumed and converted to silicon dioxide. This leads to alteration of the critical dimensions (i.e., feature size) of the trench. Also, thick shoulders of silicon dioxide may build up on the field surface and top sidewalls of the trench as a result of forming the secondary oxide layer. These thick shoulders can result in void formation during subsequent filling of the trench.
Therefore, a method of obtaining a rounded top corner on a trench formed in a semiconductor substrate which does not alter the critical dimensions of the trench and does not interfere with subsequent filling of the trench would be highly advantageous. It would also be desirable to provide a method of obtaining a rounded top corner on a semiconductor trench that can be used for both shallow trench and vertical trench applications.